Shown is a middle-stage cumulonimbus cloud with the characteristic anvil top. Cumulonimbus, or thunderstorm clouds can produce rain, thunder, lightning, and if conditions are right, tornadoes. As the updrafts-rising pockets of warm, moist air-reach warmer air at the top of the troposphere, the top of the cloud spreads out and flattens. Rain or hail starts to fall when the updrafts no longer have the energy to carry the heavy precipitation. Thunderstorms can last from one to several hours. Courtesy of UCAR Digital Image Library